Abstract

We demonstrate a compact and stable source of polarization-entangled pairs of photons, one at 810 nm wavelength for high detection efficiency and the other at 1550 nm for long-distance fiber communication networks. Due to a novel Sagnac-based design of the interferometer no active stabilization is needed. Using only one 30 mm ppKTP bulk crystal the source produces photons with a spectral brightness of 1.13 × 106 pairs/s/mW/THz with an entanglement fidelity of 98.2%. Both photons are single-mode fiber coupled and ready to be used in quantum key distribution (QKD) or transmission of photonic quantum states over large distances.

Figures (4)

Principle of the Sagnac interferometer. Depending on the polarization of the entering pump photon (green) the loop is passed in either clockwise (a) or counterclockwise (b) direction. The pump photons are transformed into signal (red) and idler (purple) by SPDC. The pump or the signal and idler photons are polarization flipped in cases a and b, respectively.

Setup for efficiency measurement and quantum state tomography. The signal photon at 810 nm is detected by a Si-APD detector. A trigger signal is sent to an InGaAs-APD detector where the appropriately delayed 1550 nm idler photon can be detected. Quarter-wave plates and polarizers in both arms are used for the characterization of the polarization state.